This invention relates generally to stripline baluns that can be implemented in surface mount packages and more particularly to a symmetrical balun that can be fabricated in a package whose shape can be selected to conform to the requirements of a microwave circuit whose arrangement is affected by other constraints.
A balun is a passive three port electronic circuit that can be used for conversion between symmetrical (balanced) and non-symmetrical (unbalanced) transmission lines.
Baluns can be implemented in a variety of ways using different types of transmission line elements. At low frequencies, and less frequently at high frequencies, coaxial transmission line segments can be used to form baluns. For example, a quarter wavelength of coaxial cable having its outer conductor grounded at the single ended side, and an input applied to the single ended end of the quarter wave length cable will produce a balanced output between the cable conductors at the opposite end of the cable. A balanced signal applied to the non-grounded end will produce a single ended output at the grounded end. Although the performance of baluns constructed from coaxial cable is acceptable, at high frequencies the difficulties associated with accurately cutting the coaxial cable to the required length affects performance. Also, coaxial cable baluns are difficult to integrate with the other elements of microwave circuits where surface mount techniques are widely used, and are not well suited for high volume production.
Printed circuit forms of baluns have been used in an attempt to overcome some of these problems. In U.S. Pat. No. 4,193,048 a balun transformer made from stripline elements formed on a printed circuit board is described. The balun transformer is fabricated from a pair of conductors each having first and second ends located on opposite sides of the printed circuit board. The first end of each conductor is located adjacent its second end. This type of balun, while an improvement over coaxial cable baluns in very high frequency applications, is difficult to employ in high power applications, because of the proximity of large metal heat sinks associated with the amplifying transistors. Inevitably, one side of the circuit board is located closer to the metal heat sinks than the other, causing an unacceptable imbalance in paracidic capacitances applied to the balun.
U.S. Pat. No. 5,061,910 attempts to provide an improved printed circuit balun that includes a plurality of serially connected first conductor elements, preferably a contiguous merged conductor extending between a single ended signal port and ground, and a plurality of second conductor elements, also preferably in the form of a contiguous merged conductor coupled to the first conductor elements and electrically isolated therefrom, the second conductor elements extending in electrical symmetry from ground to a balanced port, the first and second conductor elements being separated by an electrical isolation layer, preferably the dielectric layer of the printed circuit board.
While an improvement over earlier circuit board baluns, the balun described in U.S. Pat. No. 5,061,910 nevertheless requires a special package which although compatible with high power transistors, is not compatible with surface mount techniques.
U.S. Pat. No. 5,697,088 describes a more recent configuration of stripline elements to form a balun useful at very high frequencies, but little is said about the construction of the balun, and no consideration of the desirability of providing the balun suitable for use in surface mount circuit arrangements appears.
U.S. Pat. No. 5,644,272 shows a balun having both distributed (stripline) elements and discrete elements combined in a multi-layer dielectric structure. The resulting balun is somewhat more complicated than desirable, and although described as being automatically mountable, does not address the particular requirements of surface mounting.
Surface mount components are designed to be mounted on a printed circuit board having printed circuit traces on at least one surface of the board. A surface mount component has terminals that are connected to the printed circuit traces by soldered connections between the terminals and the printed circuit traces. Unlike non-surface mount techniques, surface mount components do not include leads that extend through holes in the printed circuit board. Surface mount components are particularly well suited to automatic assembly. The components including the balun of this invention are mounted on continuous tapes formed into reels that are used by the automatic assembly equipment to place the components on the printed circuit board. Typically, the components are temporarily attached to the board with an adhesive, solder paste, or the like prior to soldering, and then soldered in a single operation with the other surface mount components. In order to permit the surface mount component to be heat sinked, the paracidic capacitance problems of printed circuit baluns of the type described before must be overcome.
In surface mount applications, it is desirable to provide a circuit arrangement that can be implemented in a package that can be configured in a shape compatible with the other elements of the circuit. It is desirable to provide the balanced input ports and the unbalanced output port of the balun on opposite ends of the surface mount package. It is also desirable to provide a balun that can have a symmetrical physical shape that can be adjusted in length and width to physically fit within a particular circuit arrangement.